Repeating group is not supported in this simplistic scheme, but could be accommodated. I can think one simple and one robust design:

Design 1: array<list<string>>, so a regular non-repeating tag will hit a singular list.

Design 2: we ought to know which tags are repeating. The message specification must tell us which tag (NoRoutingIds in the example) signals the arrival of a repeating group. Therefore, at compile time (not runtime) system knows what tags are special i.e. potentially repeating. We will build a separate data structure for them, such as a list<groupMember>. So each FIX message will have one regular array + one list